Continental Drift, Sea Floor Spreading and Plate Tectonics 4/13/2017

Noticing the Continents About 400 years ago explorers were sailing the oceans and examining the coastlines. These explorers brought back enough information for mapmakers to create reliable maps. In 1620, Francis Bacon noticed that the shapes of some of the continents looked as if they could fit together. He proposed that they once fit together but he didn’t have a way to explain it. Everyone rejected his idea! REJECTED

What continents look like they fit together best?

Continental Drift…the beginning Everything was once connected! In 1912, a German meteorologist came up with a theory that was similar to Francis Bacon’s. His name was Alfred Wegener. He called his theory, the “Theory of Continental Drift.” In this theory, he said that not just Africa and South America were once connected but, all the continents were once connected and have since moved apart. He called this giant land mass (supercontinent) PANGAEA. This word means “All land.”

Evidence for Continental Drift The continents of South America and Africa look like they fit together.

Evidence for Continental Drift Fossils of animals and plants were found on multiple continents.

Evidence for Continental Drift Mountain ranges “matched up” between different continents. The mountain ranges on the East Coast of North America and Northern Europe were made of the same rock and the rock was the same age.

Evidence for Continental Drift When the continents are put together to form Pangaea, the remains of glacial materials fit together to form a pattern like the large ice sheets that cover our poles today. 

Evidence for Continental Drift Coal usually forms in tropical climates. Coal can be found in areas of North America which could not support coal formation. Coal forms in Tropical climates. Similar rock layers were found across multiple continents.

Theory of Continental Drift REJECTED Theory of Continental Drift

A New Idea: Sea Floor Spreading After Wegener’s death, more clues were discovered that provided the reason why the plates moved. It was the invention of echo sounding devices (sonar) that gave insight as to what the ocean floor looked like.

Echo Sounding

Sea Floor Spreading Scientist found that the ocean floor contained mountain ranges with a steep, narrow valley running down its center. These underwater mountain ranges were all connected and spread through the center of most oceans. The range when all connected were 65,000 km long. In 1947, scientists went to map the Mid-Atlantic ridge. While studying the ridge, the scientists collected and studied rock samples. They found something astonishing. The ocean floor was younger than the continents.

The Mid-Ocean Ridge

Sea Floor Spreading The scientists wanted to know two things: How did the mid-ocean ridges form? Why where the rocks younger in the ocean crust when compared to the continental crust? Write a hypothesis as to why the ocean crust is younger than the continental crust.

Sea Floor Spreading In 1960, Dr. Harry Hess came up with some answers. A driving force, convection currents, moved the plates. The ocean floor, and the rock beneath it, are produced by magma that rises from deeper levels. The hot, less dense magma rises and cools, pushing the older crust out from the center.

Convection Currents

Sea-Floor Spreading At the mid-ocean ridge, molten material rises from the mantle and erupts. The molten material spreads out, pushing older rock to both sides of the ridge. This process that continually adds to the ocean floor is sea floor spreading.

Evidence for Sea Floor Spreading In the 1960s scientists found evidence that new material is erupting along the mid-ocean ridge. The presence of pillow shaped rocks has showed that molten material has erupted again and again.

Pillow Lava rocks

Evidence for Sea-Floor Spreading Evidence in the rocks shows that Earth’s magnetic poles have reversed. The iron particles lined up in the opposite direction.

Evidence for Sea-Floor Spreading

Evidence for Sea-Floor Spreading

Evidence for Sea-Floor Spreading In the 1968, scientists aboard the Glomar Challenger gathered information about the Sea floor. They found rocks no older than 180 million years.

Evidence of Sea-Floor Spreading Evidence from drilling into the sea floor shows evidence of the ages of rocks. The closer you get to the mid-ocean ridge the younger the rock. The farther you get, the older the rock.

Subduction The processes of subduction and sea-floor spreading can change the size and shape of the oceans. Because of this processes, the ocean floor is renewed about every 200 million years.

Subduction in the Oceans The Pacific Ocean is shrinking! This is due to the fact that a deep-ocean trench is swallowing more crust than the mid-ocean ridge can produce. The Atlantic Ocean however, is expanding!

Continental vs. Oceanic Crust Type of Crust Thickness Density Composition Ocean Continental

Plate Tectonics The theory of plate tectonics combines the theories of Continental drift and Sea floor spreading. The earth’s crust is not all in one piece. It is divided in about a dozen major “plates” that ride on the earth’s asthenosphere.

Divergent Plate Boundary

Convergent Plate Boundary

Transform Plate Boundary

Ocean-Continent Convergent

Ocean-Ocean Convergent

Continent-Continent Convergent